Cellular, molecular and physiological studies have made major contributions toward a mechanistic understanding of PEPT2 structure, function and localization. However, these experimental approaches are often limited by an in vitro design and lack of blood supply, overlapping substrate specificities, and the contribution of multiple transport systems, some of which are unknown at the time of study. As a result, it is difficult, if not impossible, to define the function of a single specific gene product and its significance in relation to other possible proteins that are present in the tissue or organ of interest. The long-termobjectives of this competing renewal application are to define the physiological and pharmacological roles and relevance of PEPT2, a nutrient and drug transporter. Our working hypothesis is that PEPT2 is the primary transporter responsible for the disposition and dynamics of peptides and peptide-like drugs within the body, particularly the brain and kidney. To test this hypothesis, the following specific aims are proposed: Aim 1. To define the role and relative importance of PEPT2 in affecting the in vivo pharmacokinetics, tissue distribution and systemic exposure of peptides and peptide-like drugs; Aim 2. To determine the regional influence of PEPT2 on peptide efflux from the cerebrospinal fluid and its influence on pharmacologic response in the brain; Aim 3. To characterize the cellular localization and functional activity of PEPT2 in the brain parenchyma of developing mice. By combining membrane, cellular, immunolocalization and whole animal studies in wild type and PEPT2 null mice, the proposed studies will greatly advance our understanding of the in vivo role, significance and vectorial transport of peptide substrates and drugs by PEPT2 (as opposed to other potential transporters). These studies may also provide rare insight into the variability of peptide/mimetic kinetics and response in those human subjects with genetic polymorphisms. Finally, the proposed studies may have important implications for the design, delivery and targeting of peptide-based Pharmaceuticals to the brain for severe CNS disorders. Lav Description: This project will determine how a specific transporter, PEPT2, affects the disposition of peptides and peptide-like drugs in the kidney and brain. Results from these studies will improve the safety and efficacy of drugs for infection, hypertension and cancer, and facilitate the development of new drugs for brain disorders (e.g., Alzheimer's disease, AIDS).